Astrobiology is a new interdisciplinary science that studies the origin, evolution, distribution, and destiny of life in the cosmos. Other terms that have been used to describe the search for life beyond Earth include exobiology, exopaleontology, and bioastronomy. Astrobiology is a broadly based, interdisciplinary science that embraces the fields of biology and microbiology, microbial ecology, molecular biology and biochemistry, geology and paleontology, space and gravitational biology, planetology, and astronomy, among others.
The development of astrobiology as a discipline began in the early 1990s with the recognition of a growing synergy between various sciences in seeking answers to the question of extraterrestrial life. The National Aeronautics and Space Administration (NASA) promoted the development of astrobiology by funding a research institute (the NASA Astrobiology Institute, or NAI), which consists of interdisciplinary teams of scientists from fifteen separate institutions in the United States, including both government laboratories and universities. Important scientific discoveries have changed the way scientists think about the origin, evolution, and persistence of life on Earth. These discoveries have helped fuel the growth of astrobiology by defining the broad conceptual framework and scope of the field and by opening up new possibilities for the existence of extraterrestrial life.
Earth's Microbial Biosphere
Since the late 1980s, advances in genetics and molecular biology have radically altered scientists' view of the biosphere and the contribution of microbial life to planetary biodiversity. The opportunity to compare gene sequences from a wide variety of living organisms and environments has shown that living organisms cluster into one of three biological domains: the Archaea, Bacteria, or Eukarya. Each of these domains is made up of dozens of biological kingdoms, the vast majority of which are microbial. Species inferred to be the most primitive forms so far discovered are all found at high temperatures (greater than 80°C [176°F]) where they use simple forms of chemical energy. However, knowledge of Earth's biodiversity is still very much a work in progress. While biologists have sampled a wide range of environments, it is estimated that only a small fraction, perhaps 1 to 2 percent of the total biodiversity present, has so far been captured. Still, the three-domain structure has remained stable. New organisms are being discovered each year, adding diversity to each domain, but many discoveries still lie ahead.
These advances in biology have led to a growing awareness that Earth is overwhelmingly dominated by microscopic life and that these simple forms have dominated nearly the entire history of the biosphere. Indeed, advances in paleontology have now pushed back the record of microbial life to within half a billion years of the time scientists believe Earth first became inhabitable. This suggests that once the conditions necessary for life's origin were in place, it arose very quickly. Exactly how quickly is not yet known, but in geologic terms, it was a much shorter period than previously thought. This view significantly improves the possibility that life may have originated on other planets such as Mars, where liquid water may have been present at the surface for only a few hundred million years, early in the planet's history.
The Evolution of Complex Life
Studies of the fossil record have revealed that complex, multicellular forms of life (plants and animals) did not appear on Earth until about 600 million years ago, which is recent in geological history. Animals are multicellular consumers that require oxygen for their metabolism. Scientists believe that their late addition to the biosphere was triggered by the buildup of oxygen in the oceans and atmosphere to a threshold of about 10 percent of the present atmospheric level.* It is clear that the high level of oxygen found in the atmosphere today could have been generated only through photosynthesis, a biological process that captures sunlight and uses the energy to convert carbon dioxide and water to organic matter and oxygen. Clearly, oxygen-evolving photosynthesis has had a profound effect on the biosphere. If oxygen was required for the appearance of complex animal life, then a detailed understanding of photosynthetic processes and their evolution is crucial to create a proper context for evaluating the cosmological potential for life to evolve to the level of sentient beings and advanced technologies elsewhere in the cosmos. This research also provides a context for the SETI program (Search for Extraterrestrial Intelligence), which is currently exploring the heavens for advanced civilizations elsewhere in the galaxy by monitoring radio waves.
Basic Requirements for Life
The most basic requirement of living systems is liquid water, the universal medium that organisms use to carry out the chemical reactions of metabolism. Water is a unique dipolar compound (positively charged on one side and negatively charged on the other) with special solvent properties that allow it to act as a universal medium of transport and exchange in chemical reactions. In addition, the physical properties of water allow it to remain liquid over a very broad range of temperatures, thus enhancing its availability to living systems. In exploring for life elsewhere in the cosmos, the recognition of the importance of liquid water as a requirement for life is reflected in NASA's basic exploration strategy, which seeks to "follow the water."
But to exist, living systems also require sources of nutrients and energy. The common biogenic elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur), which comprise the basic building blocks of life, appear to be widely distributed in the universe. These elements are forged in the interiors of stars through nuclear fusion reactions, and through normal processes they produce elements with masses up to that of iron—56. The heavier metallic elements, some of which living systems also require, are formed only in very massive stars during supernova explosions. A key question of astrobiology concerns the distribution of massive stars in galaxies, which in turn may control the distribution of heavy elements essential for life.
By applying new methods of molecular biology and genetics over a broad range of environmental extremes, scientists' knowledge of the environmental limits of life on Earth (and the ways that organisms obtain nutrients and energy) has expanded dramatically. This area of inquiry comprises a relatively new area of biology known as extremophile (extreme-loving) research. This research has revealed that microbial species thrive in environments with broad extremes of temperature, ranging from deep-sea,hydrothermal vents (about 114°C [237°F]) to Siberian permafrost (−15°C [5°F]). (Above about 130°C [266°F], complex organic molecules become unstable and begin to break down. This temperature may comprise an absolute upper limit for life based on the limitations of carbon chemistry.) In addition, microorganisms occupy nearly the entire pH range from about 1.4 (extremely acid) to about 13.5 (extremely alkaline). Microbial life also occupies an equally broad salinity range from freshwater to saturated brines (containing about 300 percent dissolved solids) where salt (NaCl) precipitates. Finally, organisms also survive at very low water availability by creating desiccation -resistant structures that can survive for prolonged inclement periods.
Alternative Energy Sources
Within the basic constraint of liquid water, barriers to life appear to be few. However, it is important to understand that the level of productivity possible for living systems is strictly constrained by the quality of the energy sources they are able to exploit. On Earth, more than 99 percent of the energy powering the biosphere is derived from photosynthesis. This is not surprising given that, per unit area of Earth's surface, energy from the Sun is several hundred times more abundant than the thermal and chemical energy sources derived from within Earth. Clearly, there is a great advantage (energetically speaking) in exploiting solar energy. But the potential importance of chemical sources was also made clear in 1977 when American oceanographers Jack Corliss and Robert Ballard piloted the deep submersible,Alvin, to hydrothermal springs on the seafloor located more than 2.4 kilometers (1.5 miles) deep. At this depth, no sunlight exists for photosynthesis, and yet complex ecosystems were found there in which the organisms (including large, multicelled animals) derived their energy entirely from chemical sources provided by the hot fluids. This discovery shocked biologists, as they realized that even though photosynthesis provides much more energy, simple forms of chemical energy are still capable of supporting complex ecosystems. Since 1977, many other examples of deep-sea vent ecosystems have been found in virtually every ocean basin on Earth.
A Deep Subsurface Biosphere
As methods of exploration and observation have improved, life's environmental limits have continued to expand. In 1993 American biochemist Thomas Gold suggested that single-celled forms of life survive and grow in the deep subsurface of Earth, residing within tiny pore spaces and fractures in indurated rocks . In fact, volumetrically, such subsurface life forms could comprise more than half of Earth's biomass. Microscopic life is also thought to exist in a deep subglacial lake called Vostoc, which lies more than 3 kilometers (1.9 miles) beneath the ice cap of Antarctica. While many subsurface microbes appear to depend on photosynthetically derived organic matter that washes down from the surface, some species can make their own organic molecules from inorganic sources. Called lithoautotrophs (which literally means "self-feeding on rocks"), these organisms use the byproducts of simple weathering processes in which carbon dioxide dissolved in groundwater reacts with rocks to yield hydrogen. Hydrogen in turn is exploited for available energy. These organisms hold special importance for astrobiology because their existence allows the possibility that subsurface life can exist completely independently of surface (photosynthetic) production. Such lifestyles hold important implications for Mars and Europa (one of Jupiter's largest moons), where deep subsurface habitats are postulated to exist.
Studies of extremophiles have revealed that terrestrial life occupies virtually every imaginable habitat where liquid water, chemical nutrients, and simple forms of energy coexist. This observation has dramatically expanded the range of habitats available to life as well as the potential for life elsewhere in the solar system or beyond.
Exploring for a Martian Biosphere
Liquid water is unstable in surface environments on Mars today, thus imposing a formidable barrier to the development and survival of Martian life. Nevertheless, models suggest that a global groundwater system could exist on Mars today at a depth of several kilometers below the surface. Indeed, the Viking orbiters revealed many ancient channel features on Mars that formed when groundwater escaped and flooded onto the surface. But could groundwater still exist there today? In 2001 planetary scientists Michael Malin and Kenneth Edgett, using a high resolution camera onboard the Mars Global Surveyor mission, detected more than 140 sites on Mars where water appears to have seeped out of the subsurface, carving small channels in the surface. Under current conditions, average crustal temperatures on Mars are well below the freezing point of freshwater almost everywhere on the surface. Such surface springs of liquid water, however, could be sustained by warm, saline brines (salt lowers the freezing point of water) derived from deep hydrothermal sources. If this hypothesis is proven, the presence of liquid water—even hot, salty water—will substantially enhance the biological potential of Mars.
On Earth, scientists have found fossil biosignatures in sedimentary rocks going as far back as there are sedimentary sequences to sample. By studying the processes that govern the preservation of fossil biosignatures in similar environments on Earth, scientists are continuing to refine their understanding of the factors that govern fossil preservation. This provides a basis for the strategic selection of sites on Mars to explore with future landed missions and for sample returns. Due to the lack of plate tectonic recycling and extensive aqueous weathering on Mars, rocks preserved in the heavily cratered, ancient highlands appear to extend back to the earliest history of the planet. The rocks of these old crustal regions could be much better preserved on Mars than they are on Earth. In fact, a meteorite of Martian origin (ALH 84001), which has been dated at about 4.56 billion years, shows very little evidence of aqueous weathering.
Searching for Life in the Outer Solar System
The discovery that life can survive in deep subsurface environments on Earth, where no sunlight exists, has dramatically reshaped the ways scientists think about the potential for subsurface life on other planets. In the outer reaches of the solar system, energy from sunlight is inadequate to maintain the temperatures required for liquid water at the surface, much less for photosynthesis. However, where internal heat sources exist, liquid water could in principle be present in the subsurface.
Three of the larger satellites of Jupiter (Io, Europa, and Ganymede) appear to possess actively heated interiors that are maintained by gravitational tidal forces. These forces continually distort the shapes of these moons, creating internal friction that is capable of melting rock. In one of Jupiter's satellites, Io, the internal heating is manifested as widespread, active volcanic activity at the surface. On Europa, however, interior heating is manifested in a complexly fractured and largely uncratered (constantly renewed) outer shell of water ice. In many places, blocks of crust have drifted apart and liquid water or warm ice has welled up from below and frozen out in between, forming long, narrow ridges in the spaces between. Over time, some ridge segments have shifted laterally, offsetting older ridge segments along faults . Other more localized areas appear to have melted over broad regions and blocks of ice have foundered, tilted, and become refrozen. At an even finer scale, there are smaller, mounded features that are thought to have formed as ice "volcanoes" erupted water or warm ice erupted water from the subsurface.
While the concept of a Europan ocean is still controversial, measurements of the magnetic field of the moon obtained during the Galileo mission have strengthened the case. In order to account for the induced magnetism measured by Galileo, it is likely that a salty ocean exists beneath the water ice crust. (Similar arguments have also been made for two other large satellites of Jupiter, Ganymede and Callisto.) The idea of an ocean of brine beneath the icy crust is consistent with infrared spectral data from orbit, which suggest that magnesium and/or sodium sulfate salts are present in surface ices.
In assessing the potential for life on Europa, the presence of liquid water is regarded as crucial, both as a medium for biochemical processes and as a source for the chemical energy necessary to sustain life. There does not appear to be enough solar energy at the surface of Europa to support life. However, in 2001 planetary scientist Chris Chyba proposed a model that predicts that chemical energy sources for supporting life may exist from radiation processing of Europa's surface ice, in combination with the decay of radioactive potassium. Together, these processes could decompose water to hydrogen and oxygen (with the hydrogen escaping to space) and the chemical disequilibrium created potentially exploited for energy by organisms.
Habitable Environments Beyond the Solar System
The discovery of planets orbiting other Sun-like stars in the galaxy is a key scientific discovery that has played a central role in the astrobiological revolution. The original discoveries, made in the mid-1990s, have continued. By the early twenty-first century,extrasolar planets have been found orbiting almost seventy solar-mass stars in the nearby region of the galaxy. Six of these discoveries are of planetary systems with two or more planets. Present discovery methods are based on the detection of a slight shift or "wobble" in the position of the star that results from the gravitational pull of an orbiting planet(s). With existing technologies, this method allows for the detection of planets that are Jupiter-sized or larger. Some of the extrasolar planets detected occupy orbits within the habitable zone where liquid water could exist. Gas giants (such as Jupiter and Saturn) are planets that lack a solid surface, but they could contain interior zones of liquid water, or might have large (undetectable) satellites with solid surfaces and liquid water. These discoveries have revealed planets around other stars to be commonplace in the Milky Way, thus widening the possibilities for life elsewhere in the cosmos.
see also Extrasolar Planets (volume 2); Jupiter (volume 2); Mars (volume 2); Mars Missions (volume 4); Planetary Protection (volume 4); Scientific Research (volume 4); SETI (volume 2); Terraforming (volume 4).
Jack D. Farmer
Chang, Sherwood. "The Planetary Setting of Prebiotic Evolution." In Early Life on Earth, ed. S. Bengston. New York: Columbia University Press, 1994.
Chyba, C., and K. Hand. "Life without Photosynthesis."Science 292 (2001):2,026-2,027.
Fredrickson, J. K., and T. C. Onstott. "Microbes Deep Inside the Earth."Scientific American 275, no. 4 (1996):42-47.
Klein, H. P. "The Search for Life on Mars: What We Learned from Viking."Journal of Geophysical Research 103 (1998):28,463-28,466.
Lemonick, Michael D. Other Worlds: The Search for Life in the Universe. New York:Simon & Schuster, 1998.
Pappalardo, R. T., J. W. Head, and R. Greeley. "The Hidden Ocean of Europa."Scientific American (October 1999):34-43.
*Oxygen currently makes up 21 percent of Earth's atmosphere.
"Astrobiology." Space Sciences. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/astrobiology
"Astrobiology." Space Sciences. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/astrobiology
Astrobiology: Water and the Potential for Extraterrestrial Life
Astrobiology: Water and the Potential for Extraterrestrial Life
Astrobiology is a new interdisciplinary science that seeks to understand the origin, evolution, distribution, and future of life in the universe. As a fundamental requirement of living systems, water holds a special place in the conceptual framework of astrobiology. All of life's processes are carried out in the presence of liquid water, and on this basis it may be regarded as a key indicator for potential habitability. The importance of liquid water as an organizing principle in the exploration for extraterrestrial life often is articulated in the simple expression "follow the water."
Water and Planetary Habitability
What is it about water that justifies its central role in the search for extraterrestrial life? Most of water's unique properties (e.g., its excellent solvent properties, broad temperature range over which it remains liquid, high heat capacity, and surface tension) are rooted in the ability of water molecules to form hydrogen bonds with each other. In addition, on freezing, there is a slight expansion of hydrogen bond angles that produces a solid phase (ice) of lower density than the liquid phase. This uncommon property results in waterbodies that freeze from the top downward, an important factor for sustaining habitability in polar and other cold climates.
Clearly, a knowledge of the past and present distribution of water in the solar system is regarded as crucial for evaluating the potential of other planets (or their moons) to develop and sustain life. Water also holds central importance in the human exploration of the solar system, being essential for the colonization of other planets, such as Mars.
Throughout Earth's history, water has played a central role in the global cycles that link the solid Earth and the atmosphere. Interactions between crustal rocks and water sustain a broad range of processes that collectively meet most of the important energy and resource requirements of living systems. Such interactions ultimately determine the overall habitability of a planet, thus setting the stage for life's origin and ensuring its persistence over geologic timescales.
The hydrologic cycle (the cycling of water between the atmosphere and oceans) drives a vast transport system that constantly redistributes materials and energy within the Earth's crust. Flowing water and ice transport rock fragments and associated weathering products from source areas to basins of deposition. Streams and groundwater (inclusive of hydrothermal systems) dissolve, transport, and concentrate chemical compounds required by organisms.
Sediments and the dissolved materials formed during weathering processes ultimately reach the ocean basins , where they accumulate as dissolved salts or seafloor sediments. Over the long term, even the dissolved load of streams eventually precipitate out of solution as secondary minerals (such as sedimentary cements) and chemical sediments (such as evaporites ). The deposits so formed often preserve signals for environmental change on Earth along with a fossil record of life's evolution.
Over longer spans of time, cycling of the crust by the subduction of lithospheric plates and melting of sediment-covered seafloor and entrapped sea water produce magmas (molten rock materials). The water dissolved in these magmas actually lowers their density and crystallization temperature, thus promoting their buoyant rise back to the surface, where they drive volcanic activity.
Over geologic timescales, volcanic outgassing of the Earth's interior regulates atmospheric composition and evolution.
The Earth's close orbital distance from the Sun ensures a vast supply of solar energy that is utilized by photosynthetically based surface ecosystems. However, the energy output of the Sun was probably much lower (30 percent less than present luminosity) at the beginning of solar system history.
Under these relatively faint young-Sun conditions, an atmospheric greenhouse, sustained by carbon dioxide (CO2) and/or methane (CH4), was required to maintain habitable surface conditions. An active plate tectonic cycle over the entire history of Earth has allowed for the constant renewal of the atmosphere by volcanic outgassing. This atmospheric renewal is essential for long-term sustainability. (By contrast, see the discussion of Mars farther ahead in this entry and elsewhere in the encyclopedia.)
By approximately 2.5 billion years ago, interactions between the global hydrologic system and geologic cycles of the solid Earth (via processes such as plate tectonics, weathering and erosion, and volcanism) had produced a clear compositional differentiation of the Earth's habitable surface environments into two broad habitats: the continental land masses and the ocean basins. Around the same time, oxygenic photosynthesis emerged as a major biological innovation, taking advantage of the abundant energy available from the Sun. Oxygen production through photosynthesis eventually outstripped volcanic and weathering controls on atmospheric composition, producing an oxidizing surface environment.
By approximately 600 million years ago, the buildup of oxygen in the atmosphere culminated in the appearance of large, multicellular life forms. This new level of organization in the biosphere enhanced global biodiversity , leading in stepwise fashion to the emergence of terrestrial (land-based) faunas and eventually to intelligent life characterized by self-awareness and advanced cultural, social, and technological civilizations.
Exploring for Martian Life
Given the terrestrial experience of humans, it is easy to understand why the search for water in all its forms, past or present, has emerged as the primary theme for exploration of the solar system. For example, over the next decade, scientific efforts to explore for water on Mars will create a context for assessing planetary habitability and the potential for Mars having developed life at some time in its history.
Presently, the surface of Mars is properly regarded as a radiation-rich frozen desert that is hostile to life. Within about 1 billion years of its origin, Mars appears to have lost most of its atmosphere and, with that, the potential for sustaining liquid water environments at the surface. Interestingly, this early loss of the atmosphere appears to have been the result of the absence of a plate tectonic cycle on Mars.
Yet Mars has not always been a dry, hostile place. Exploration efforts in the late twentieth century revealed that prior to the loss of its atmosphere, Mars probably was much more Earth-like. The ancient southern highlands of Mars harbor a wide variety of water-carved landforms and layered sedimentary deposits of likely aqueous origin. The broad temporal distribution of these features suggests that even though the surface of Mars has been dry for most of the planet's history, liquid water has been present from time to time, providing brief intervals of surface habitability.
Loss of the Martian atmosphere would have spelled doom for any surface life existing at the time. However, if Martian life forms colonized surface environments during earlier wet periods, they are quite likely to have left behind a fossil record. The search for this fossil record is in many ways the focus of the current Mars exploration program.
The possibility of living Martian life-forms is one facet of ongoing research. On Earth, scientists have discovered that life occupies an incredible range of environmental extremes, including the deep subsurface, where it utilizes chemical energy instead of sunlight. Models suggest that liquid water (perhaps saline) environments could still exist today in the deep subsurface of Mars, along with energy-containing compounds such as methane, which could sustain chemically based life. The argument for subsurface habitability is strengthened by the existence of ancient out-flood channels, believed to have been formed by catastrophic releases of subsurface water in the past. These landforms provide direct evidence that a groundwater system once existed.
But what about today? Scientists recently discovered what appear to be water-carved gullies on the steep slopes and high latitudes of Mars. Despite the constant subfreezing temperatures at those latitudes, water, in the form of subsurface hydrothermal brines , may have risen from deep crustal sources along faults , flowing briefly over the surface and carving the channels.
The origin of these seep features remains controversial, but the hydrologic interpretation is consistent with a variety of other types of evidence that suggest the presence of a subsurface groundwater system. Further investigation of these features is warranted. If a subsurface groundwater system does exist on Mars, such environments may have provided stable habitats for life over the entire history of the planet. In 2002, the gamma-ray spectrometer onboard NASA's Odyssey orbiter discovered extensive water present as ground ice in surface soils over extensive regions of Mars at high latitudes. This has strengthened the case for an abundance of subsurface crustal water on Mars.
In exploring for Martian groundwater, the practical problem faced by NASA (National Aeronautics and Space Administration) is accessibility. Accessing and sampling sources of subsurface Martian water (and potentially life) will require the development of precision landing systems capable of safely landing on steep slopes where potential seep sites are located, and/or long-ranging rovers capable of traveling to prospective groundwater sites (such as seeps) from safe landing sites located at a distance of perhaps tens of kilometers. Next, scientists will need to drill to depths of tens to hundreds of meters from small robotic platforms, a capability they presently lack.
Although the previously mentioned technological capabilities have all been identified as long-term goals of NASA's Mars exploration program, scientists presently lack the technologies needed to access subsurface water on Mars with robotic platforms. As a result, some have suggested that drilling for Martian groundwater may require a human presence, something that is beyond the scope of the present Mars program. The earliest human missions to Mars, if they can be safely carried out, are unlikely to occur prior to 2025.
see also Comets and Meteorites, Water in; Earth: The Water Planet; Fresh Water, Physics and Chemistry of; Life in Extreme Water Environments; Mars, Water on; Solar System, Water in the; Volcanoes and Water.
Jack D. Farmer
Carr, M. H. Water on Mars. London, U.K.: Oxford University Press, 1996.
Chang, S. "The Planetary Setting of Prebiotic Evolution." In Early Life on Earth, ed. S. Bengston. New York: Columbia University Press, 1994.
Klein, H. P. "The Search for Life on Mars: What We Learned from Viking." Journal of Geophysical Research. 103 (1998):28463–28466.
Lemonick, M. D. Other Worlds: the Search for Life in the Universe. New York: Simon and Schuster, 1998.
Malin, M. C., and K. S. Edgett. "Evidence for Recent Groundwater Seepage and Surface Runoff on Mars." Science 288 (2000):2330–2335.
Pace, N. R. "A Molecular View of Microbial Diversity and the Biosphere." Science 276 (1997):734–740.
LIFE IN A MARTIAN METEORITE?
About 20 percent of the magnetites found in a 4.6-million-year-old Martian meteorite named ALH84001 resemble intracellular magnetites formed by some species of terrestrial bacteria. (Magnetite is a naturally magnetic mineral common in basalt.) Whether the meteoritic magnetites are a reliable indicator of life was under scientific scrutiny as of 2002.
"Astrobiology: Water and the Potential for Extraterrestrial Life." Water:Science and Issues. . Encyclopedia.com. 23 Aug. 2017 <http://www.encyclopedia.com>.
"Astrobiology: Water and the Potential for Extraterrestrial Life." Water:Science and Issues. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/astrobiology-water-and-potential-extraterrestrial-life
"Astrobiology: Water and the Potential for Extraterrestrial Life." Water:Science and Issues. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/astrobiology-water-and-potential-extraterrestrial-life
astrobiology: see exobiology.
"astrobiology." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/astrobiology
"astrobiology." The Columbia Encyclopedia, 6th ed.. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/astrobiology